[ARM] pxa: workaround errata #37 by not using half turbo switching

PXA27x Errata #37 implies system will hang when switching into or out of
half turbo (HT bit in CLKCFG) mode, workaround this by not using it.

Signed-off-by: Dennis O'Brien <dennis.obrien@eqware.net>
Cc: stable-2.6.31 <stable@kernel.org>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>